The sun’s violent death could look like this

The sun still has a long life ahead of it, say five billion years or so. But our star is still on borrowed time. When its inevitable end arrives, the sun’s core will exhaust the last traces of hydrogen fuel and kick off the first stellar death pangs. At that point, our yellow sun will start swelling into a red giant star about 100 times its current size. Over the next one to two billion years, its outer layers will begin shedding as it transitions into its final phase as a white dwarf. The closest planetary neighbors—Mercury, Venus, and possibly even Earth—will be destroyed in the resultant collateral damage.

But what about the fate of the solar system’s five other known orbiting planets? The answer is much murkier, especially for the gas giants. To learn more about our cosmic neighborhood’s potential distant future, astronomers conducted a case study on a recently discovered gas giant located about 80 light-years from Earth. Their findings, published today in the journal Nature, reveal surprising insights into what the future of life may look like in the solar system. That is, if any lifeforms remain.

“Our results show that stellar death is not the end—some planets experience a vibrant and lively future after the death of their star,” Ryan MacDonald, a University of St. Andrews astronomer and study co-author, said in a statement.

WD 1856 b is about the size of Jupiter. Credit: European Space Agency

Cosmic life after death

The cosmic subjects in question are the white dwarf WD 1856+534 and its orbiting gas giant, WD 1856 b, which MacDonald describes as “quite the oddball.”

“It’s about the size of Jupiter, but the white dwarf it orbits is the size of Earth, so the planet is seven times larger than its star,” he explained.

Astronomers first spotted WD 1856 b using the Spitzer Space Telescope and NASA’s Transiting Exoplanet Survey Satellite (TESS) in 2020. The gas giant circles its star at an unexpectedly close distance—about 50 times nearer than the Earth’s orbit around the sun. It’s such a strange feature that it actually marks the first known example of an intact planet surviving that close to a white dwarf. But if WD 1856 b had always orbited that near its star, the gas giant would have undoubtedly been obliterated during the red giant era.

So this begs an important question about how WD 1856 b even got there in the first place. According to Northwestern University astronomer Christopher O’Connor, there are currently two main theories.

“One is that the planet was swallowed by the host star as it was dying, and managed to survive on the inside,” he said. “The other is that the migration took place due to the gravitational effect of other objects in the system.”

Those “other objects” are the two other stars in WD 1856+534’s triple star system. Its stellar companions potentially may have also influenced WD 1856 b’s orbit at some point in its history.

Then there is the matter of the planet’s temperature, which was assessed with help from the ever-busy James Webb Space Telescope (JWST). Using the JWST, researchers observed WD 1856 b partially overlap its star during what’s known as a grazing transit. This allowed them to analyze crucial information about not only the gas giant’s temperature, but also its mass and atmospheric chemical composition.

By using spectroscopy, the light that passed through the planet’s atmosphere during the transit can be split up into its constituent wavelengths. Doing this shows that some wavelengths are missing, because they were absorbed by molecules in the atmosphere and blocked, while other wavelengths are transmitted without being blocked. This “transmission spectrum” therefore shows which molecules are present in the planet’s atmosphere. Credit: ESA / Ryan MacDonald

Heated attraction

The results suggest the planet exhibits a temperature of 400 Kelvins, or about 260 degrees Fahrenheit. That’s much hotter than it should be if its only heat source is a white dwarf. Because there are no other nearby heat generators, that energy must be residual aftereffects of an earlier era. The planet either warmed up while engulfed during the red giant phase, or began heating as gravity pulled it closer to the resultant white dwarf.

Although the additional heat was perplexing, it also proved a critical detail in determining how WD 1856 b got so close to its star. By combining the planet’s mass data with its current temperature, astronomers calculated things started heating up 3 to 5.5 billion years after its star transitioned into a white dwarf. This means the exoplanet originally resided safely outside the red dwarf’s diameter.

“As the planet moved inwards, its interactions with the strong gravity of the white dwarf will have caused it to warm up considerably, and it has been cooling ever since,” said O’Connor.

Beyond its travel history, the team also detected signs of molecules still inside its atmosphere. JWST readings revealed clear signatures of tiny cloud particles and hydrocarbons like methane—another first for an exoplanet orbiting a dead star.

A time machine telescope

WD 1856 b still has a lot to tell astronomers, and they’re ready to keep digging. Even more atmospheric data will soon become available from four additional recent transits by JWST. The entire project highlights the mindbending beauty and potential history lessons from peering deep into the cosmos.

“It’s like using a time machine to peer into the distant future of our solar system,” said MacDonald.

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